105
100
95
90
85
T0 T1 T2
V3
V2
V1
V4
+%
- %
V1 = Voltage dip T0 = Point at which load is appliedV2 = Maximum
transient voltage overshoot T1 = Time to recover to a specic bandV3
= Recovery voltage T2 = Time to recover to and remain within the V4
= Steady-state regulation specied band
TIME (SECONDS)
VOLT
AGE
(PER
CEN
T of
RAT
ING
)
Genset transient voltage versus time for sudden load change
Recovery voltage orsustained voltage dip (V3)
Instantaneousvoltage dip (V1)
1.0 Introduction
When starting electric motors with a standby generator, it is
important to understand the effect of the voltage dip when
calculating the sizing criteria. This information sheet discusses
the effects voltage dip can have on an electrical system and
considerations that should to be taken when designing and
specifying an electrical system with motor loads.
2.0 Instantaneous Voltage Dip (IVD)Most manufacturers of
alternators and generator sets measure for the instantaneous
voltage dip as the primary factor for motor load sizing. Both
National Electrical Manufacturers Association (NEMA) and DOD
Military (MIL) standards also recognize the initial voltage dip as
the primary criteria.
To fulfill our commitment to be the leading supplier in the
power generation industry, the Loftin Equipment and Bay City
Electric Works teams ensures they are always up-to-date with the
current power industry standards as well as industry trends. As a
service, our Information Sheets are circulated on a regular basis
to existing and potential power customers to maintain their
awareness of changes and developments in standards, codes and
technology within the power industry.The installation information
provided in this information sheet is informational in nature only
and should not be considered the advice of a properly licensed and
qualified electrician or used in place of a detailed review of the
applicable National Electric Codes, NFPA 99/110 and local codes.
Specific questions about how this information may affect any
particular situation should be addressed to a licensed and
qualified engineer and/or electrician.
Your Reliable Guide for Generator MaintenanceConsidering
Instantaneous Voltage Dip Versus Sustained Voltage Dip
Information Sheet # 18
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78148Ph: 210.881.1623Fx: 210.881.2143Ph: 866.441.0375 Toll Free
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BCEW
/LEC-
INFO
#18
2013
PLC
Ente
rpris
es, L
LC
Examples of IVD and its consequence are Represents the actual
maximum voltage dip experienced when an induction motor load is
applied.
Maximum voltage dip due to motor inrush current normally occurs
within 5 cycles and then quickly recovers with the automatic
voltage regulator (AVR) sensing the drop and increasing current
flow to the alternator field.
Industry generally regards a 30% to 35% IVD as the standard to
start a motor, and the proper operation of the other equipment
connected to the generator.
Generally only between 1 and 4 seconds lapse before recovery
time but this can be extended dependant on the motor load and
genset combination.
Any larger % dip could cause problems with sensitive loads and
result in adverse issues such as starter motor chatter and /or drop
out, some static UPS units to malfunction, overheating, extended
load acceleration times, circuit breakers or other protective
devices to open and engine-generator shutdowns to operate.
3.0 Sustained Voltage Dip (SVD)This is also called the recovered
voltage or the voltage level that the generator recovers to after
the instantaneous dip. The sustained voltage value is determined by
applying stepped load increases until the alternator does not
recover to 90% of sustained voltage.
4.0 Why is 90% SVD Considered This recognizes that over-current
protection devices are affected by the heat in the system. As such,
it defines the concept of continuous loads and the 90% rule in
order to try and offset the effects of heat in the system when
sizing a CB.
5.0 Comparing IVD with SVDBy definition IVD and SVD are very
different. The following should be noted when comparing IVD and
SVD: With no direct relationship between IVD and SVD - any
correlation can be misleading in performance terms. Their maximum
KVA values are different. Maximum starting KVA (SKVA) based on a
sustained voltage of 90% can show a much larger value, but the
voltage dip is almost always much greater than 35%. Specifications
should call for instantaneous voltage dip value for SKVA at a given
voltage dip for a consistent comparison thereby ensuring correct
operation of sensitive loads
6.0 When to Consider Evaluating IVD or SVD
Instantaneous Voltage Dip (Why it is normally a primary
consideration for motor starting systems) Motor starting is
recognized as a complete system performance issue with IVD
considered the primary criteria influencing a motor starting
system. Ignoring the complexities and dynamics of IVD, without the
proper prototype tests and system modeling, can result in an
improperly sized generator. If a large motor system can tolerate
long start times the initial voltage dip is less important.
However, it is important to review alternator damage curves to
determine if high inrush currents, when starting large,
slow-starting motors and will exceed the equipments design
limits.
Sustained Voltage Dip (Why it is considered a secondary criteria
for motor starting systems) SVD is calculated by ignoring the
initial instantaneous dip despite most industry experts (including
NEMA and MIL standards) considering it to be the primary criteria.
As such, only considering SVD is not a comprehensive system
performance solution or approach when calculating equipment sizes
in a motor starting system. It significantly understates the actual
voltage dip experienced by equipment and motors connected to the
generator, which can result in motor starters dropping out and
other equipment malfunctions due to a higher IVD exceeding the
maximum SKVA calculated for the application. If the sustained SKVA
is larger than the instantaneous voltage dip SKVA value, then the
sustained SKVA must be ignored as the motor starter would drop out
and not see the additional SKVA.
7.0 Fundamentals for Successful Motor Starting. Sufficient IVD
and SKVA the motor rotates, starter contacts hold in and no other
loads affected Sufficient generator set torque the motor rotates
and does not stall out Sufficient HP/kW and exciter forcing motor
accelerates and reaches operational speed and voltage
Note! The maximum KVA at 90% sustained voltage values are larger
than instantaneous SKVA at 35% voltage dip8.0 Key Recommendations
and Conclusions. A) SVD and IVD values are not the same and should
therefore not be compared B) Neither approach should be the ONLY
method used for motor starting sizing calculations C) Instantaneous
VD is used as the primary criteria for motor starting sizing D)
Sustained VD SKVA cannot exceed IVD SKCA at the required voltage
dip level (SVD SKVA can overstate the maximum KVA) E) Use the IVD
SKVA value in specifications for proper motor starting and
operation of other connected equipment F) Trust the sizing software
of major manufacturers for the dynamics of motor starting
